1. Field of the Invention
The present invention relates to a method for producing seed crystals used for a method for producing nickel powder by adding seed crystals to an acid solution and blowing hydrogen gas to reduce the solution.
2. Description of the Related Art
Examples of the method for smelting nickel include a method of roasting an ore into the form of a sulfide or an oxide and reducing the sulfide or the oxide to obtain ferronickel which is an alloy with iron to provide it as a raw material for stainless steel, and a method of separating impurities from an acid solution in which a sulfide is dissolved in hydrochloric acid or sulfuric acid and electrowinning the acid solution to obtain electric nickel. Further, a nickel salt such as nickel sulfate and nickel chloride may be recovered from the acid-dissolved solution and used for plating, a battery material, and the like.
In addition, examples of a method for producing nickel powder in a powder state from the nickel salt include a hydrometallurgical process shown in Japanese Patent Laid-Open No. 2005-240164.
The method of Japanese Patent Laid-Open No. 2005-240164 is a so-called complexing reduction method including mixing a nickel sulfate aqueous solution with a complexing agent to subject the mixture to complexing treatment to form a nickel ammine complex solution, putting the solution in a pressurized container, sealing the container, heating the solution to about 150 to 250° C. followed by maintaining the temperature, and blowing hydrogen gas into the solution, in which the nickel ammine complex is reduced by hydrogen to produce nickel powder.
The above method of reduction using hydrogen gas allows an industrially stable operation and is a production method suitable for mass production.
However, when metallic particles are intended to be obtained by reduction reaction, there has been a problem that, if a reducing agent added and a solution do not uniformly react with each other, crystal nuclei is produced at random to produce an excessively fine powder, and it is difficult to obtain uniform particles to reduce the reduction efficiency.
Further, when a gas such as hydrogen gas as described above is blown into a solution, partially non-uniform growth is highly likely in the solution since the bubbles of the gas physically has a certain or larger particle size.
In the production of nickel powder, a reduction efficiency of about 80% or more is industrially required. If the reduction efficiency is excessively low, for example, less than 80%, there will be too much loss, and repeated production will be required, which is not preferred.
Therefore, there is used a method in which seed crystals are previously mixed in a solution and nickel particles are grown using the seed crystals as the nuclei. Since properties such as the size and shape of seed crystals to be used greatly influence the growth of particles, it is necessary to use uniform seed crystals.
Therefore, although there is also a method of repeatedly using a part of the product as seed crystals, the method has a problem that the time and efforts are required to process the product into sizes and properties suitable for seed crystals; and the yield is reduced since a part of the product once produced is repeatedly used, thus increasing the cost.
Therefore, it is industrially expected to use, as seed crystals, iron powder or a compound of iron which can be produced in a large amount as a product having uniform quality, and in Japanese Patent Laid-Open No. 2005-240164, an iron compound is added as seed crystals during reduction reaction to precipitate nickel on the iron compound.
However, there has been a problem that, since iron powder is used, the iron is mixed into the product; and therefore, it is difficult to use iron as seed crystals for the application requiring high purity quality.
On the other hand, Japanese Patent Laid-Open No. 2005-240164 and disclose methods of obtaining nickel powder using a reducing agent other than hydrogen gas.
Japanese Patent Laid-Open No. 2005-240164 provides nickel powder which is inexpensive, is excellent in weatherability, has low electric resistance in a state where it is kneaded with a resin, reduces initial electric resistance and electric resistance in use, can be stably used over a long period of time, and is suitable as conductive particles for a conductive paste and a conductive resin, and a method for producing the same. Specifically, the nickel powder disclosed in Japanese Patent Laid-Open No. 2005-240164 contains 1 to 20% by mass of cobalt with the balance consisting of nickel and unavoidable impurities, comprises secondary particles in which primary particles are aggregated, and has an oxygen content of 0.8% by mass or less. Cobalt is contained only in the surface layer of the secondary particles, and the cobalt content in the surface layer is preferably 1 to 40% by mass.
However, since an impurity such as cobalt is added as seed crystals also in the method of Japanese Patent Laid-Open No. 2005-240164, the quality of the product may be reduced by the mixing of cobalt into the product in the same manner as in the case of using iron as seed crystals.
Further, Japanese Patent Laid-Open No. 2010-242143 discloses a method for producing metal powder by a liquid phase reduction method that is improved so that a particle aggregate may be hardly produced.
The method for producing metal powder comprises a first step of dissolving a metal compound, a reducing agent, a complexing agent, and a dispersant to prepare an aqueous solution containing metal ions derived from the metal compound, and a second step of adjusting the pH of the aqueous solution to reduce the metal ions with the reducing agent to precipitate the metal powder.
Seed crystals obtained by this wet reaction method is preferred because it can precisely provide a powder having a primary particle size of 0.5 to 5.0 μm which is generally considered as the powder that can be most easily utilized as seed crystals. However, the method disclosed in Japanese Patent Laid-Open No. 2010-242143 has not been practical because the reagents such as a reducing agent and a dispersant to be added are expensive and have many problems in terms of cost for industrial use in a large amount. Particularly, when hydrazine is used as a reducing agent, there has also been a problem of an increase in environmental load such as an increase in wastewater treatment load in addition to an increase in cost.
As described above, there has been desired a method for producing nickel seed crystals that maintains and improves the quality of nickel powder at a low cost.
The present invention provides a method for producing nickel seed crystals that maintains and improves the quality of nickel powder at a low cost while suppressing the production cost and environmental load in the production of nickel powder, by optimizing the amount of hydrazine added when producing fine nickel powder as seed crystals using hydrazine, and a method for producing nickel powder using the nickel seed crystals.